Changing ASCII case using bitwise operator

I am writing this post to share the method with my friends.

I am doing Bachelor in Computer Science and Engineering. My microprocessor course teacher gave us a task to change case from upper or lower to uppercase.

It is kind of "DO it yourself" task.

I am using emu 8086 for this task.

Here are two test cases that we have must pass:

Teas Case 1:
Input : abcd
Output : ABCD

Teas Case 2:
Input : aBCd
Output : ABCD

NOTE: Input will be always 4 character and between a-z or A-Z.

We can solve it using conditional instruction.
To make the problem more interesting, I've challenged myself to solve it without branching.

Gladly, I've found a way to solve it using bitwise operator.

We can see that the 6th bit is zero for uppercase character and one for lowercase character.

If we can set the bit to 0 no matter what is the input, the character will become Uppercase.

We can set it to zero and left rest of bits unchanged by using bitwise AND operation.

a ->   0110 0001
       1101 1111
----------------
AND -> 0100 0001

I am going to use 11011111 (binary) or DF (hex) to set the 6th bit to zero and leave other bits unchanged.

PROC MAIN
   MOV AH, 01H   ; input mode

   INT 21H       ; take 1st character in AL
   AND AL, 0DFH  ; perform AND operation
   MOV BL, AL    ; Move result to another Register

   INT 21H       ; same as 1st character
   AND AL, 0DFH
   MOV BH, AL

   INT 21H       ; same as 1st character
   AND AL, 0DFH
   MOV CL, AL

   INT 21H       ; same as 1st character
   AND AL, 0DFH
   MOV CH, AL


   MOV AH, 02H

   MOV DL, 0AH   ; print newline
   INT 21H
   MOV DL, 0DH   ; print cret
   INT 21H 


   MOV DL, BL    ; print 1st character
   INT 21H

   MOV DL, BH    ; print 2nd character
   INT 21H

   MOV DL, CL    ; print 3rd character
   INT 21H

   MOV DL, CH    ; print 4th character
   INT 21H

ENDP

Case 1 :

Case 2 :

Happy Coding !!!